Controlled release systems deliver a drug at a predetermined rate for a definite time period, that may range from days to years. These systems provide advantages over conventional drug therapies. For example, after ingestion or injection of standard dosage forms, the blood level of the drug rises, peaks, and then declines. Since each drug has a therapeutic range above which it is toxic and below which it is ineffective, oscillating drug levels may cause alternating periods of ineffectiveness and toxicity. In contrast, a controlled release preparation maintains the drug in the desired therapeutic range by a single administration. Other potential advantages of controlled release system include: (i) localized delivery of the drug to a particular body compartment, thereby lowering the systemic drug level; (ii) preservation of medications that are rapidly destroyed by the body (this is particularly important for biologically sensitive molecules such as proteins); (iii) reduced need for follow up care; (iv) increased comfort; and (v) improved compliance.
Optimal control of drug release may be achieved by placing the drug in a polymeric material. Polymeric materials generally release drugs by diffusion, chemical reaction, or solvent activation.
The most common release mechanism is diffusion, whereby the drug migrates from its initial position in the polymeric system to the polymer's outer surface and then to the body. Diffusion may occur through a reservoir, in which a drug core is surrounded by a polymer film, or in a matrix, where the drug is uniformly distributed through the polymeric system. Drugs can also be released by chemical reaction such as degradation of the polymer or cleavage of the drug from a polymer backbone.
Combinations of the above mechanisms are possible. Release rates from polymeric systems can be controlled by the nature of the polymeric material (for example, crystallinity or pore structure for diffusion controlled systems; the hydrolytic lability of the bonds or the hydrophobicity of the monomers for chemically controlled systems) and the design of the system (for example, thickness and shape). The advantage of having systems with different release mechanisms is that each can accomplish different goals.
For many years, controlled release systems were only capable of slowly releasing drugs of low molecular weight (&lt;600). Large molecules, such as proteins, were not considered feasible candidates, because polypeptides were considered too large to slowly diffuse through most polymeric materials, even after swelling of the polymer. The discovery that matrices of solid hydrophobic polymers containing powdered macromolecules enabled molecules of nearly any size to be released for over 100 days permitted controlled delivery of a variety of proteins, polysaccharides, and polynucleotides. See Langer, 1990.
The proteins and polypeptides incorporated up to this date in polymeric materials for controlled release are mainly effector molecules, such as insulin, as opposed compositions for controlled release of molecules that will bind and neutralize effector molecules produced in the human body.
Tumor necrosis factor-.alpha. (TNF.alpha.) is a potent cytokine which elicits a broad spectrum of biological responses. TNF.alpha. is cytotoxic to many tumor cells and may be used in the treatment of cancer. TNF.alpha. enhances fibroblast growth and acts as a tissue remodeling agent, being thus suitable in wound healing. It further induces hemorrhagic necrosis of transplanted tumors in mice, enhances phagocytosis and cytotoxicity of polymorphonuclear neutrophils, and modulates the expression of many proteins, including lipoprotein lipase, class I antigens of the major histocompatibility complex, and cytokines such as interleukin-1 and interleukin-6. TNF.alpha. has been shown to have an effect against virus, bacteria and multicellular, particularly intracellular, parasites. TNF.alpha. appears to be necessary for a normal immune response, but large quantities produce dramatic pathogenic effects. TNF.alpha. has been termed "cachectin" since it is the predominant factor responsible for the wasting syndrome (cachexia) associated with neoplastic disease and parasitemia. TNF.alpha. is also a major contributor to toxicity in gram-negative sepsis, since antibodies against TNF.alpha. can protect infected animals.
TNF.alpha. has been shown to be involved in several diseases, examples of which are adult respiratory distress syndrome, pulmonary fibrosis, malaria, infectious hepatitis, tuberculosis, inflammatory bowel disease, septic shock, AIDS, graft-versus host reaction, autoimmune diseases, such as rheumatoid arthritis, multiple sclerosis and juvenile diabetes, and skin delayed type hypersensitivity disorders.
Evidence that some of the effects of TNF.alpha. can be detrimental to the host have attracted attention to the mechanisms that regulate TNF.alpha. function. The intracellular signals for the response to TNF.alpha. are provided by cell surface receptors (herein after TNF-R), of two distinct molecular species, to which TNF.alpha. binds at high affinity.
The cell surface TNF-Rs are expressed in almost all cells of the body. The various effects of TNF.alpha., the cytotoxic, growth-promoting and others, are all signalled by the TNF receptors upon the binding of TNF.alpha. to them. Two forms of these receptors, which differ in molecular size, 55 and 75 kilodaltons, have been described, and will be called herein p55 and p75 TNF-R, respectively. It should be noted, however, that there exist publications which refer to these receptors also as p60 and p80 TNF-R.
Both receptors for TNF.alpha. exist not only in cell-bound, but also in soluble forms, consisting of the cleaved extracellular domains of the intact receptors, derived by proteolytic cleavage from the cell surface forms. These soluble TNF.alpha. receptors (sTNF-Rs) can maintain the ability to bind TNF.alpha. and thus compete for TNF.alpha. with the cell surface receptors and thus block TNF.alpha. activity. The sTNF-Rs thus function as physiological attenuators of the activity of TNF.alpha., safeguarding against its potentially harmful effects. It has, however, also been reported that the sTNF-Rs affect TNF.alpha. function also by stabilizing its activity, most likely by preventing dissociation of its bioactive trimeric structure to inactive monomers (Aderka et al., 1992). Thus, the sTNF-Rs may affect TNF.alpha. activity in two different ways: either they compete for TNF.alpha. with the cell surface receptors and block TNF.alpha. deleterious effects, or they act as buffering agents and stabilize TNF.alpha. activity.
The two sTNF-Rs, hereinafter p55 sTNF-R and p75 sTNF-R, have been formerly designated TNF Binding Proteins I and II, or TBPI and TBPII, respectively (see, for example, EP 398327, EP 412486, and EP 433900). In the present application we will use both designations: p55 sTNF-R or TBPI and p75 sTNF-R or TBPII; by either designation the proteins are the same.
The sTNF-Rs are present constitutively in serum at concentrations that increase significantly in both inflammatory and non-inflammatory disease states. The effect of these proteins may differ, however, depending on their concentrations at the site of TNF.alpha. action, the relation of their concentration to the local concentration of TNF.alpha., and the rates at which the sTNF-Rs and TNF.alpha. are cleared from the site of TNF.alpha. action in relation to the rate of decay of TNF.alpha. activity. Dependent on these parameters, the sTNF-Rs may, in different situations, affect the function of TNF.alpha. in quite a different manner, either by inhibiting the effects of TNF.alpha., or serving as carriers for TNF.alpha. or even augmenting the effects of TNF.alpha. by prolonging its function (Aderka et al., 1992).
The effectivity of the sTNF-Rs as anti-TNF.alpha. drugs can be affected by a number of different factors: The affinity at which the sTNF-Rs bind TNF.alpha., compared to the affinity of the cell-surface receptors, the accessibility of the soluble receptors to the site of TNF.alpha. action and the rate of the clearance of the soluble receptors and of the complexes which they form with TNF.alpha. from the site of TNF.alpha. formation. The natural forms of the sTNF-Rs are likely to act in the most physiologically relevant manner. However, a major limitation in their use is their rather rapid clearance from the blood. Several attempts have been made to improve these molecules, examples of which are the so-called chimeric "immunoadhesins", in which the sTNF-Rs are linked to the Fc portion of the immunoglobulin molecule (developed by Hoffman La Roche and Immunex), and "PEGulated" sTNF-Rs, in which the sTNF-Rs are cross-linked through PEG molecules (developed by Synergen). Both approaches result in formation of divalent sTNF-R molecules which have longer clearance time and can bind more effectively to the trivalent TNF.alpha. molecule. However, they are quite likely to be more immunogenic than the natural soluble receptors, and the clearance of their complex with TNF.alpha. from the circulation may not occur at a sufficient efficiency.
Many harmful effects of TNF.alpha. result from chronic formation of this cytokine at certain distinct loci in the body. A major foreseen limitation to the use of soluble forms of TNF receptors for defense against such pathological conditions is the difficulty in maintaining therapeutically effective concentration of the soluble receptors, for prolonged durations, at sites of need.